1. Field of the Invention
This invention relates to high frequency solid state transistors, and more particularly to Group III nitride based field effect transistors and high electron mobility transistors.
2. Description of the Related Art
Microwave systems commonly use solid state transistors as amplifiers and oscillators which has resulted in significantly reduced system size and increased reliability. To accommodate the expanding number of microwave systems, there is an interest in increasing their operating frequency and power. Higher frequency signals can carry more information (bandwidth), allow for smaller antennas with very high gain, and provide radar with improved resolution.
Field effect transistors (FETs) and high electron mobility transistors (HEMTs) are common types of solid state transistors that are fabricated from semiconductor materials such as Silicon (Si) or Gallium Arsenide (GaAs). One disadvantage of Si is that it has low electron mobility (approximately 1450 cm2/V-s), which produces a high source resistance. This resistance seriously degrades the high performance gain otherwise possible from Si based FETs and HEMTs. [CRC Press, The Electrical Engineering Handbook, Second Edition, Dorf, p.994, (1997)]
GaAs is also a common material for use in FETs and HEMTs and has become the standard for signal amplification in civil and military radar, handset cellular, and satellite communications. GaAs has a higher electron mobility (approximately 6000 cm2/V-s) and a lower source resistance than Si, which allows GaAs based devices to function at higher frequencies. However, GaAs has a relatively small bandgap (1.42 eV at room temperature) and relatively small breakdown voltage, which prevents GaAs based FETs and HEMTs from providing high power at high frequencies.
Improvements in the manufacturing of GaN/AlGaN semiconductor materials have focussed interest on the development of GaN/AlGaN based FETs and HEMTs. These devices can generate large amounts of power because of their unique combination of material characteristics including high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The same size GaN amplifier can produce up to ten times the power of a GaAs amplifier operating at the same frequency.
U.S. Pat. No. 5,192,987 to Khan et al discloses GaN/AlGaN based HEMTs grown on a buffer and a substrate, and a method for producing them. Other HEMTs have been described by Gaska et al., xe2x80x9cHigh-Temperature Performance of AlGaN/GaN HFET""s on SiC Substrates,xe2x80x9d IEEE Electron Device Letters, Vol. 18, No 10, October 1997, Page 492; and Ping et al., xe2x80x9cDC and Microwave Performance of High Current AlGaN Heterostructure Field Effect Transistors Grown on P-type SiC Substrates,xe2x80x9d IEEE Electron Devices Letters, Vol. 19, No. 2, February 1998, Page 54. Some of these devices have shown a gain-bandwidth product (fT) as high as 67 gigahertz (K. Chu et al. WOCSEMMAD, Monterey, Calif., February 1998) and high power densities up to 2.84 W/mm at 10 GHz (G. Sullivan et al., xe2x80x9cHigh Power 10-GHz Operation of AlGaN HFET""s in Insulating SiC,xe2x80x9d IEEE Electron Device Letters, Vol. 19, No. 6, June 1998, Page 198; and Wu et al., IEEE Electron Device Letters, Volume 19, No. 2, Page 50, February 1998.)
Despite these advances, GaN/AlGaN based FETs and HEMTs have been unable to produce significant amounts of total microwave power with high efficiency and high gain. They produce significant power gain with DC gate drives, but with frequency step-ups as low as a millihertz to a few kilohertz, their amplification drops off significantly.
It is believed that the difference between AC and DC amplification is primarily caused by surface traps in the device""s channel. Although the nomenclature varies somewhat, it is common to refer to an impurity or defect center as a trapping center (or simply trap) if, after capture of one type carrier, the most probable next event is re-excitation. In general, trapping levels located deep in a band gap are slower in releasing trapped carriers than other levels located near the conduction of valence bands. This is due to the increased energy that is required to re-excite a trapped electron from a center near the middle of the band gap to the conduction band, compared to the energy required to re-excite the electron from a level closer to the conduction band.
AlxGa1-xN (X=0xcx9c1) has a surface trap density comparable to the channel charge of the transistor with the traps in deep donor states with activation energy ranging from approximately 0.7 to 1.8 eV (depending on X). During FET and HEMT operation, the traps capture channel electrons. The slow trapping and de-trapping process degrades transistor speed, which largely degrades the power performance at microwave frequencies.
The present invention provides an improved Group III nitride based FETs and HEMTs that are preferably formed of GaN/AlGaN and exhibit improved amplification characteristics in response to AC gate drives. The invention also provides a new method for producing the new GaN/AlGaN FET and HEMT.
The new FET comprises a barrier layer on a high resistivity, non-conducting layer. Source, drain and gate contacts are included, with each contacting the barrier layer. A electron donor layer is formed on the surface of the barrier layer between the contacts, the donor layer preferably being a dielectric layer with a high percentage of donor electrons.
For the new HEMT, the barrier layer has a wider bandgap than the non-conducting layer and, as a result, a two dimensional electron gas (2DEG) forms at the junction between the barrier layer and the non-conducting layer. The 2DEG has a high concentration of electrons which provide an increased device transconductance. The new HEMT has contacts that are similar to those on the FET""s conducting channel and a similar dielectric layer is included on the HEMT""s conducting channel.
In each device, it is believed that the barrier layer has surface traps that are positively charged. It is also believed that the electron donor layer""s donor electrons migrate to the device""s barrier layer and fill the surface traps. This causes them to become neutral and prevents them from capturing free electrons. The new electron donor layer also increases sheet electron density in the un-gated regions of the devices and protects the devices from undesirable passivation, impurities and damage during handling.
The present invention also provides a method for producing the new GaN FET or HEMT. The new method relies on sputtering techniques and results in little to no damage to the surface of the conduction channel. It also provides a strong and stable bond between the dielectric layer and the surface of the channel.
These and other further features and advantages of the invention would be apparent to those skilled in the art from the following detailed description, taking together with the accompanying drawings, in which: